Scholarly article on topic 'Tandem organic photodetectors with tunable, broadband response'

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Academic research paper on topic "Tandem organic photodetectors with tunable, broadband response"

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Tandem organic photodetectors with tunable, broadband response

S. Matthew Menke, Richa Pandey, and Russell J. Holmes

Citation: Appl. Phys. Lett. 101, 223301 (2012); doi: 10.1063/1.4768807 View online: http://dx.doi.org/10.1063/1.4768807 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v101/i22 Published by the American Institute of Physics.

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Tandem organic photodetectors with tunable, broadband response

S. Matthew Menke, Richa Pandey, and Russell J. Holmes

Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, USA

(Received 29 September 2012; accepted 9 November 2012; published online 27 November 2012)

Broadband photodetection is achieved by integrating three electron donor materials with complementary absorption into an organic photodetector (OPD). While a single donor-acceptor heterojunction can show broadband response, the spectral tunability is intrinsically limited to the absorption profiles of the active materials. Here, we demonstrate broadband OPDs consisting of multiple bulk heterojunctions arranged in tandem. These OPDs show high responsivity under moderate reverse bias from the ultraviolet to the near-infrared. By combining materials with complementary absorption in a tandem OPD, we demonstrate that the response from each band can be separately tuned with manipulation of the heterojunction thicknesses or composition. © 2012 American Institute of Physics. [http://dx.doi.org/10.1063/L4768807]

Organic photodetectors (OPDs) are of interest for a wide range of sensing applications including imaging, communications, surveillance systems, and biological sensing.1,2 Photodetectors based on organic semiconductors are attractive for their compatibility with flexible substrates and broad, tunable absorption characteristics. Recent demonstrations have shown that broadband response can be realized in OPDs using a variety of organic1,3-7 and hybrid organic-inorganic8 systems. In the case of organic semiconductors, photodetection requires the efficient dissociation of optically generated excitons. Under a low applied field, this dissociation occurs at the interface between electron donating and accepting materials. Multilayer OPDs sensitive in the visible spectrum incorporating planar electron donor-acceptor (D-A) heterojunctions in series have been previously used to realize high quantum efficiencies and short response times.9-11 Mixtures of D-A species, forming bulk heterojunctions, can also be employed; however, the response times are generally slower.3,12

Commonly, broadband response in an OPD is achieved by forming planar or bulk heterojunctions that incorporate a narrow energy gap electron donor material paired with a full-erene acceptor material. While these simple structures can exhibit high external quantum efficiencies and optical responsivities, the spectral tunability of these devices is intrinsically limited to the absorption profiles of the respective donor and acceptor materials. In fact, previous reports of spectral tunability refer to methods that serve only to shift the wavelength of peak response via optical engineering13 or materials selection.14 In order to realize both broadband response and inter-band spectral tunability, it is necessary to incorporate additional active materials such that the overall OPD response is the superposition of multiple narrow absorption profiles instead of a single broad absorption profile. Here, we examine broadband OPDs incorporating three donors having complementary absorption extending into the near-infrared (NIR), permitting spectral tunability without sacrificing broadband response. We realize this integration in a single photodetector by stacking, in tandem, multiple D-A bulk heterojunction photodetectors, with each sub-cell tuned

to a different absorption band. Additionally, this device architecture does not require highly conductive recombination layers between the stacked bulk heterojunctions, which are common in many tandem organic photovoltaic cells and light-emitting devices. We show that the spectral responsiv-ity of each band can be separately tuned by simple manipulation of the heterojunction thicknesses. This approach permits the realization of a broadband photodetector whose spectral response is highly tunable and can be tailored depending on the application.

Organic photodetectors were fabricated on glass substrates coated with a 150-nm-thick layer of indium-tin oxide (ITO) having a sheet resistance of 15 Q/d. Substrates were degreased and cleaned with solvents and treated in UV-ozone ambient prior to film deposition. All organic layers were grown using vacuum thermal evaporation (<10"7 Torr) at a nominal rate of 0.2nm/s. A 10-nm-thick interlayer of MoO3 was deposited on the ITO substrates prior to deposition of the organic active layers to reduce device dark current. Broadband OPDs were fabricated using three electron donor materials with complementary optical absorption: boron subphthalocyanine chloride15 (SubPc), chloroalu-minum phthalocyanine16 (ClAlPc), and tin naphthalocyanine dichloride5 (SnNcCl2). Each donor material is paired with the electron acceptor C60. Bulk heterojunctions were fabricated by co-deposition of the respective D-A materials at a mixing ratio of 1:1. A 10-nm-thick exciton blocking layer of bathocuproine (BCP) was deposited on top of the organic active layers. Device active areas were defined by evaporating a 65-nm-thick layer of Al through a shadow mask with 1-mm-diameter openings.

For spectrally resolved measurements of the external quantum efficiency (gEQE) and responsivity, devices were illuminated by a 300 W Xenon lamp coupled to a Cornerstone 130 1/8 m monochromator and chopped with a Stanford Research Systems SR540 optical chopper at 200 Hz. Electrical characteristics were measured using a Stanford Research systems SR810 lock-in amplifier and an Edmund Optics photodiode amplifier. Dark current characteristics were measured using an Agilent 4155C parameter analyzer.

0003-6951/2012/101 (22)/223301/4/$30.00 101, 223301-1 ©2012 American Instituteof Physics

FIG. 1. Molecular structures for the electron donating materials SubPc (a), ClAlPc (b), and SnNcCl2 (c) along with their respective absorption coefficients (d) which span the ultraviolet to the near infrared.

The donor materials of SubPc, ClAlPc, and SnNcCl2 were chosen based upon their complementary absorption spectra as determined from spectroscopic ellipsometry (Fig. 1). To characterize the performance of each donor with C60, single-donor bulk heterojunction OPDs were separately characterized. For each OPD, the active layer consisted of an 80-nm-thick 1:1 mixture of the donor and C60. Figure 2 shows the responsivity for each device at a reverse bias of —6 V, demonstrating that each OPD has a narrow wavelength range of peak response, reflecting the absorption spectra of Fig. 1(d). In order to realize broadband response in a single device, mixtures of each donor and C60 were stacked

in tandem with the following layer structure: a 22-nm-thick layer of 1:1 SubPc:C60, a 27-nm-thick layer of 1:1 ClAlPc:C60, and a 40-nm-thick layer of 1:1 SnNcCl2:C60. The device structure is summarized in Fig. 2(a). The layer thicknesses were chosen to maintain a similar optical density for each individual donor, ignoring any effects due to optical interference. The responsivity for this tandem OPD at a reverse bias of —6 V is shown in Fig. 2(b) and clearly reflects the absorption behavior of each individual donor. Optical responsivities >0.2 A/W are achieved across the visible and NIR, demonstrating the broadband response of this architecture.

The highest occupied molecular orbital energy levels (HOMOs) for SubPc and ClAlPc are 5.6 eV and 5.4eV, respectively.17,18 While the HOMO level of SnNcCl2 has not been previously measured, we expect it to be shallower than that of ClAlPc since photovoltaic cells constructed using SnNcCl2 show a significantly smaller open-circuit voltage than those constructed using ClAlPc. In order to observe efficient hole collection under zero applied bias, it is necessary to reverse the ordering of the heterojunctions with respect to the tandem OPD in Fig. 2(a), hereafter referred to as the control. Figure 3 shows the gEQE as a function of reverse bias for each absorption band for both of the control structure (Fig. 3(a)) and the reverse structure (Fig. 3(b)) where the mixtures containing SubPc and SnNcCl2 are exchanged so that the bulk heterojunction containing SnNcCl2 is closest to the anode. Figure 3(b) shows that the reversed ordering leads to a moderate amount of photomultiplication, signaled by gEQE > 100%. Photomultiplication is a gain phenomenon common in OPDs corresponding to situations where more than one electron and hole are collected per incident photon absorbed.19 It generally arises due to the presence of intrinsically or extrinsically trapped charge carriers,20 giving rise to imbalanced charge transport and the subsequent injection of additional carriers which lead to additional photocurrent. The reversed ordering shows photomultiplication at reverse biases greater than a few volts and at wavelengths of k = 360 nm and k = 740 nm, corresponding to the optical absorption bands of C60 and ClAlPc, respectively. We speculate that the photomultiplicative gain present in the reversed

BCP (10 nm)

SnNcCI2:C60 (40 nm)

CIAIPc:C60 (27 nm)

SubPc:C60 (22 nm)

Mo03 (10 nm)

ITO coated Glass

■■—i—1—i—1—i—1— ■ SubPc:C60 Mixture

• CIAIPc:C60 Mixture

a SnNcCI2:C60 Mixture

-Tandem

300 400 500 600 700 800 900 1000 1100 Wavelength (nm)

FIG. 2. (a) Device structure of the tandem, broadband organic photodetector. (b) Optical responsivity spectra for single donor devices containing mixtures of C60 and one of SubPc, ClAlPc, and SnNcCl2 (broken lines). Also shown is the responsivity spectrum for a tandem device which incorporates all three donor-acceptor mixtures (solid line).

-1 0-6-5-4 Reverse Bias (V)

FIG. 3. Single wavelength external quantum efficiency (gEQE) as a function of reverse bias for the control OPD (a) and the reversed OPD (b). The layer ordering for the control OPD is (150 nm ITO/lOnm MoO3/22nm SubPc:C60/27nm ClAlPc:C6O/40nm SnNcCl2:C6O/10nm BCP/65nm Al), whereas the reversed OPD is (150nm ITO/10nm MoO3/40nm SnNcCl2:C60/27nm ClAlPc:C60/22nm SubPc:C60/10nm BCP/65nm Al).

structure is the result of trapped carriers inside the device. In contrast, the control shows no signs of photomultiplication as the gEQE at all wavelengths plateaus to a value less than 100% under reverse bias. While the existence of such a gain mechanism clearly increases the optical response, it can also decrease the speed of the OPD and is found to be difficult to control. Since our purpose is to examine the spectral tunabil-ity of the tandem OPD architecture, we focus on the control OPD structure that does not exhibit photomultiplication.

In order to demonstrate the tunability of tandem, multi-donor OPDs, devices were constructed where the thickness of each individual bulk heterojunction is reduced by 50% relative to the control. Here, OPD-SubPc has the following layer structure: an 11-nm-thick layer of 1:1 SubPc:C60, a 27-nm-thick layer of 1:1 ClAlPc:C60, and a 40-nm-thick layer of 1:1 SnNcCl2:C60. Similarly, OPD-ClAlPc and OPD-SnNcCl2 contain bulk heterojunctions of ClAlPc:C60 and SnNcCl2:C60, respectively, whose thicknesses are reduced by half relative to the control. Figure 4 compares the respon-sivities of these structures to that of the control at a reverse bias of —6 V. In Fig. 4(a), halving the thickness of the SubPc:C60 mixed layer (OPD-SubPc) decreases the response from SubPc (k — 590 nm) likely reflecting a reduction in SubPc absorption. The enhancement in responsivity for ClAlPc (k - 740 nm) and SnNcCl2 (k - 900 nm) is likely the result of improved hole collection to the ITO anode. Similarly, OPD-ClAlPc shows a reduction in responsivity for response from ClAlPc, (k — 725 nm) again reflecting a reduction in ClAlPc absorption. The minimal changes in the SubPc and SnNcCl2 absorption peaks signal that the reduction in thickness of the ClAlPc:C60 mixed layer does not strongly alter hole collection in this architecture. OPD-SnNcCl2 shows a decrease in responsivity for the SnNcCl2 absorption band (k — 900 nm) attributed to a reduction in SnNcCl2 absorption. The decrease in the ClAlPc dominant absorption band is due to a decrease in the optical electric field at k = 740 nm in the ClAlPc:C60 layer as suggested by separate modeling of the internal optical field using a transfer matrix formalism.21 These results demonstrate that spectral

'to c o

1 1 - (a) * I i i 1 i 1 i 1 -Control - .A ......OPD-SubPc 1 1 1 1 1 * r" ■ *

i 1 : (b) 1 1 1 1 1 1 -------OPD-CIAIPc J

r » ' » V "

/ V^y / v_ ,' > V/X . V- \ A -\

1 i 1 i : (c) /.i.i i | i | i | i | i | i* | .............OPD-SnNcCI2 . 1 . 1 . 1 . 1 Hwm.

0.4 0.3 0.2 0.1

0.0 0.4 0.3 0.2 0.1 0.0 0.4 0.3 0.2 0.1 0.0

300 400 500 600 700 800 900 1000 1100 Wavelength (nm)

FIG. 4. Responsivity spectrum for the control OPD (solid line) as compared to OPDs where the indicated donor-acceptor mixture thickness is halved with respect to the control.

tunability can be easily achieved with simple consideration of the mixed layer thicknesses, HOMO level landscape, and optical field inside the device.

In order to further characterize the utility of tandem OPDs, the noise equivalent power (NEP) was calculated for the control OPD. The NEP represents the minimum incident optical power than can be detected over the noise. The inverse of the NEP is defined as the detectivity. When the shot noise from the dark current is the most significant contribution to the noise, the specific detectivity normalized for the detector area and detection frequency can be expressed as1

D*= R/(2eJd)1/2, (1)

where R is the responsivity, e is the electron charge, and Jd is the dark current. The dark current for the control OPD is 10—6 mA/cm2 at 0 V and 10—1 mA/cm2 at —3 V. Detectivities for the control OPD were calculated using Eq. (1) and are summarized in Table I for selected excitation wavelengths

TABLE I. Detectivity for the control OPD.

Wavelength (nm) 0V D* (1010 Jones) -3 V

360 340 2.7

590 800 4.3

740 990 5.3

900 100 2.2

representative of each active material. These reported values are competitive with others that have been reported for other organic and quantum dot photodetectors across the visible4 and NIR1'6'7 portions of the electromagnetic spectrum. Further increases in detectivity could be realized by reducing the dark current under reverse bias with the selection of more suitable blocking layers as well as directly optimizing for enhanced photomultiplication.

In summary, we have demonstrated broadband OPDs with tunable spectral response. These devices exhibit high gEQE and responsivity at moderate reverse biases across the visible and near-IR spectrum. Broadband response was achieved by incorporating three distinct donor materials with complementary absorption into three discrete bulk hetero-junctions without the requirement of recombination layers. Tunability was achieved by simple manipulation of the het-erojunction thicknesses coupled with a consideration for how these changes affect both the optical and charge carrier dynamics within the OPD. We propose that these structures are suitable for a wide array of photodetection applications and allow for user specified spectral response.

This work was supported primarily by Raytheon Vision Systems. Partial support was also received from the University of Minnesota NSF Materials Research Science and Engineering Center (DMR-0819885). The authors would like to thank Sigma-Aldrich Corporation (New Products R&D) for providing SnNcCl2.

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